A radiation therapy installation typically includes such equipment as a CT scanner, CT simulator software, radiation treatment planning software, a linear accelerator, a multi leaf collimator, and a portal imager. Radiation treatment planning systems and image based simulators such as CT and MRI are known in the medical arts for treatment and diagnosis of disease. For example, a radiation therapy device typically includes a gantry, which can be rotated around a horizontal axis of rotation, and a patient couch, which can be rotated about a vertical axis. A linear accelerator is located within the gantry for generating a high energy radiation beam for therapy. During treatment, the beam is directed at a particular treatment zone of a patient, which is located at or about the intersection of the two axes of gantry rotation, otherwise known as the isocenter.
It is also known to use computer-controlled, motorized, mechanical shaping of radiation beams generated by such systems to produce conformal beam shaping. For example, multi-leaf collimators (MLCs) are available from Varian, Inc. of Palo Alto, Calif., Siemens Oncology Care Systems, Inc. of Concord, Calif., and others. Such multi-leaf collimators typically incorporate radiation shielding material such as tungsten leaves to conform the radiation beam more closely to a target volume, such as a tumor near vital organs in the patient's body, without exposing the surrounding organs to harmful radiation. As a result, the dosage of radiation can be increased when compared to that administered to the patient without the MLC. Examples of such systems are set forth in U.S. Pat. Nos. 4,672,212; 5,818,902; 6,577,707; 6,459,769, and others.
Quality assurance of dosimetric functions of radiation therapy planning systems and image based simulators is mandated to ensure accurate radiation planning for medical treatment. To that end, water-based phantoms are well known in the art (e.g. Ayyangar K., et al, Experimental Verification of a Three-Dimensional Dose Calculation Algorithm Using a Specially Designed Heterogeneous Phantom, Med. Phys. 20, 1993, pp. 325-329). More recently, increasing attention has been paid to quality assurance (QA) of the nondosimetric functions of such systems. For example, the AAPM Radiation Therapy Committee Task Group 53: Quality Assurance for Clinical Radiotherapy Treatment Planning, Benedick Fraass et al; Med. Phys. 25 (10), October 1998, pp. 1773-1829 highlights areas of nondosimetric QA of treatment planning that need to be addressed. The TG-53 report specifically addresses the need for QA of image acquisition, anatomical representation, beam display, plan evaluation tools, hard copy output, and other features.
Tim Craig, Dennis Brochu and Jake Van Dyk have disclosed a phantom for the QA of many nondosimetric features of three-dimensional radiation treatment planning systems and CT simulators (see A Quality Assurance Phantom for Three-Dimensional Radiation Therapy Treatment Planning, Int. J. Radiation Oncology Biol. Phys., Vol. 44, No. 4, pp. 9555-966, 1999). The phantom of Craig et al. comprises a rotatable component to assess the display of the radiation beam graphics and CT set data manipulations, and a stationary component to assess the treatment of anatomical volumes and the conversion of CT numbers to relative electron density.
Although the system of Craig et al facilitates the implementation of a program consistent with the recommendations of TG-53, there is no provision for testing the integrity of treatment planning systems and CT simulators in the display of MLC-shaped fields on transverse or reconstructed images.